The present invention relates to new resin compositions based on phenolic resins; it relates more especially to phenolic resin compositions which are especially suitable for the manufacture of "phenolic prepreg mats".
Prepregs are mixed materials which are prepared from thermosetting resins and reinforcements and optionally fillers. The reinforcement used is in the state of fibers, such as cellulose fibers or glass fibers, especially glass fiber rovings, of a non-woven made, for example, from high molecular weight polyester or from polyvinyl chloride, of glass fibre mat or fabric made, for example, from aromatic polyamide, from glass or from asbestos. These prepreg materials have the advantage of being able to be press-molded directly without the need to prepare a prior mixture consisting of resin, catalysts, fillers and pigments. Prepregs may be divided into two categories: on the one hand the so-called "non-flowing" prepregs, and on the other hand the so-called "flowing" prepregs. The "non-flowing" prepregs generally consist exclusively of glass fiber reinforcements impregnated with a suitable resin which is brought to an appropriate stage of partial polymerization, the so-called "B" stage. At this stage, the resins have virtually ceased to be tacky but are still soluble and fusible. Such a mixed material, placed in a heated press, permits movement of the resin between the fibers and leads to a uniform distribution of the glass and the resin, after the mold is closed, the resin is completely cured. Non-flowing prepregs are manufactured from epoxy resins, phenol-formaldehyde resins and also diallyl phthalate-based polyester resins. Such materials permit especially limited applications, since they most often have the disadvantage of becoming detached when used: they are used, for example, for the manufacture of printed circuits.
The known "flowing" prepregs, most often called "prepreg mats", are prepared, up to the present time, exclusively from a single category of resins, namely polyester resins. Depending on the technique of manufacture used, two varieties of "prepreg mats" are distinguished in particular.
SMC (sheet moulding compound) prepregs are mixed materials consisting of a fabric of chopped glass fibers preimpregnated with resin containing filler, pigment and catalyst. They are presented between two thin layers of film-forming material (for example polyethylene), which enables them to be handled easily. They are then fabricated into articles by compression molding.
Bulk molding compounds (BMC) are mixtures consisting of thermosetting resin, filler, and chopped glass fibers: they are presented in bulk or in hanks ready for use.
The prepreg mat thereby obtained possesses, after heat-curing, a consistency similar to that of leather or of a waxed cloth. After storage in a curing chamber, the prepreg mat thereby obtained can be stored for approximately three months, during which period it can be used hot pressure moulding.
The major difficulty which must be overcome in order to manufacture these prepreg mats based on polyester resins is to have a resin which is sufficiently fluid to be able to impregnate the chopped glass fiber mat, but in which the rise in viscosity during the first 48 hours (after which period the viscosity should remain virtually unchanged at room temperature) is sufficiently fast to be able to obtain a prepreg mat which has the desired consistency, which should be similar to that of leather or that of a waxed cloth, as stated above. It is hence seen that there are, in practice, two mutually opposing problems to be solved. In order to solve such problems, it is known to add to the polyester resin a thickener chosen from alkaline earth metal oxides, such as magnesium oxide. The addition of such a compound to the polyester resin enables a resin to be obtained which has a desired viscosity, and also sufficient stability to enable the resin not to thicken too quickly, the shortcoming of such rapid thickening being an unsatisfactory impregnation of the chopped glass fiber mats.
It is known from French Patent Application 84/12277 of Aug. 3, 1984, to manufacture pasty compositions which are suitable for the manufacture of phenolic prepreg mats by introducing an additive, consisting of at least 20% by weight, relative to the total weight of the additive, of an alkaline earth metal metaborate, into the mixture of phenolic resins consisting of resol, filler, pigments- and hardening agents.
It might have been thought that the simple replacement of the alkaline earth metal metaborates by organic borates such as boric acid esters, for example triethyl borate or tri-2-methoxyethyl) borate, would have enabled pasty compositions of phenolic resins suitable for the manufacture of phenolic prepreg mats to be obtained. Unfortunately, this is by no means the case. It is found, in effect, that the addition of these organic boron products to the phenolic resins leads to a very rapid rise in the viscosity, which results in a gelling of the resin solution in a few days.